Biologists went back out on Tuesday to check from the mouth of the river up to Fells Point and Fort McHenry, said Jay Apperson, MDE's deputy communications director. While biologists revised upwards their original estimate that maybe 1,000 fish had died Monday, they did not see any newly dead or dying fish, he said.
Fish kills are an almost annual occurrence in the harbor, but they usually happen in warmer weather, when algae growth takes off and then consumes the oxygen fish need to breathe. Upwards of 100,000 fish died over several days in late May 2012 in a kill linked to a severe algae bloom. This die-off, in cooler than typical weather, has scientists stumped.
"The cause remains under investigation," Apperson said, "and we cannot say if or when we will determine a likely cause."
Apperson did say that since the kill seemed limited to Atlantic menhaden, state biologists do not think it stemmed from a spill or release of toxic pollution, which likely would have affected other species.
Natural conditions often are a factor in fish kills, such as extreme temperatures or sudden temperature changes, diseases or parasites, low oxygen levels or algae blooms. The cause most frequently cited by investigators is a drastic drop in dissolved oxygen in the water, often in association with an algae bloom triggered by fertilizer, animal waste and sewage washing into the waterway following a storm or sewer line break.
Apperson said said preliminary water quality testing Monday did not indicate any problem with oxygen levels in the water.
Water samples taken in the Patapsco on April 17 by the Department of Natural Resources did find a lack of oxygen in deeper water below the Key Bridge, but levels nearer the surface were good enough to sustain fish, said Bruce Michael, resource assessment director for the department.
Winds might have stirred up the river in such a way to draw the low-oxygen water to the surface, trapping and suffocating the fish, Michael suggested. But he acknowledged that "this is speculation at this point."
For more on fish kills, go here.